So basically you have answered your own question:
If you have to ensure that there's always some traffic, either base station and mobile client need to regularly ping each other, or a third party that's also connected to the network via a switch needs to ping them regularly.
If the connectivity check is done inside the base station and the mobile client, you don't need a switch. If there's a third party who does the monitoring, you need a switch. If this third party also wants to analyze network traffic, you need to mirror the network traffic to the port of the third party.
So, port-mirroring and ping-echoing are totally different things. The first applies only if there's a third party doing the monitoring, the second applies only if no constant traffic can be ensured. Nothing prevents you from using both.
- Latency: You'll get the smallest latency if you're both passively monitoring (existing traffic) and ping in small intervals. Pings may be dropped because of collisions, so you need several failed pings before you can be sure the connection is broken. But if they get dropped, there will be other traffic, so if you monitor this passively, it shouldn't be a problem.
I'm not sure what you expect with respect to security and real-time measurements. Pings and ping replies are nearly instantanous, as well as passive traffic monitoring. For security considerations, you'll need to explain the sitation in more detail.